The present invention relates to the art of welding and is specifically concerned with a method of flash butt welding. The invention may be used in resistance flash butt welding of large cross-section workpieces.
A continuous resistance flash butt welding method wherein the welding machine head moves at a constant or a rising speed in the course of fusion is widely known in welding practice.
Although such a welding method offers a high electrical efficiency, its use is restricted to welding workpieces of only certain types of materials, cross-sectional sizes and shapes; it has found practical application mainly for welding thin-sheet structures and workpieces with small compact cross-sections.
A grave disadvantage of this method, which restricts its application, lies in that the initially high thermal efficiency of the fusion process progressively drops as the workpieces to be welded are being heated; this slows down the rate of their heating and therefore increases the fusion loss of metal in attaining the required zone of heating of the end faces of the workpieces. The drop in the thermal efficiency brings about serious difficulties in welding workpieces of low heat conductivity materials as well as thick-walled and compact-section workpieces, because in such cases the thermal efficiency drops at a higher rate.
There is also known a flash butt welding method wherein the workpieces to be welded are repeatedly moved towards and away from each other (Britain Pat. Nos. 1,153,002; 984,296) or one of the workpieces is set in an oscillating motion (Japanese Pat. No. 2,162). In this method, only reciprocating movements are imparted to the workpieces at the heating stage, without continuously drawing them together. This method as well suffers from disadvantages. It features a low electrical efficiency and calls for use of machines with a high electrical capacity. In order to uniformely heat the workpieces to be welded, the faces must closely fit to each other, but even the most careful fitting of the end faces fails to eliminate their nonuniform heating over the section with this welding method.
This nonuniform heating of the workpieces results in welding defects, such as incomplete fusion and dead spots.
There is further known a resistance flash butt welding method wherein the welding machine head is moved at a constant or rising speed and in addition simultaneously oscillated along the direction of movement (USSR Inventor's Certificate) No. 226,052; British Pat. No. 1,162,073; FRG Pat. No. 1,615,324; French Pat. No. 1,517,114). The combination of a continuous movement of the workpieces being welded towards each other with a simultaneous oscillation of one of these greatly increases the welding current and the thermal efficiency of the welding process as against those of the above-mentioned continuous flash welding without oscillation. Owing to this feature, the method under consideration makes it possible to weld thick-walled and compact workpieces with a large cross-section as well as workpieces of low heat conductivity materials.
A disadvantage of this welding method is that during a certain time at the initial stage of welding, while the ends of the work-pieces to be welded are cold or slightly heated, the fusion process is characterized by a current close to the short-circuit one, a power loss for heating the secondary circuit of the welding transformer, and a low electrical efficiency; in welding thick or large cross-section workpieces, this low-efficiency initial stage of welding lasts longer.